This invention relates to a system for transmitting analog information by digital modulation methods via a communication channel, the transmission properties of which can vary in the time. The invention more particularly relates to a system of adjusting the digital coding of the analog information to the varying transmission properties.
Most of the digital communication systems use transmission channels the transmission properties of which can vary to a greater or smaller extent in the time. Depending on the size and nature of these variations and on possible shortcomings in the systems, digital errors can arise at the transmission of the information. These digital errors, depending on the design of the systems, can cause interruption in the formation transmission or result in that the information transmitted and received do not entirely agree with the information to be transmitted.
When the total distortion of a communication system transmitting analog information is defined as the difference between the analog information prior to and after the transmission, the total distortion then depends on possible distortion from digital errors at the transmission and on quantizing distortion from the coding of the analog information to digital signals.
The quantizing distortion is determined by the nature of the analog information transmitted by the system and by the way this information is coded to digital signals. With the knowledge of certain statistic parameters of the information to be transmitted, the quantization distortion from the use of a certain digital coding can be calculated theoretically. The quantization distortion, therefore, in principle does not vary with the transmission properties of the channel being used, as long as the coding is not changed.
When the transmission properties of a transmission channel deteriorate beyond a certain limit, generally the number of digital errors at the transmission increases. This implies in general that the distortion from digital errors at the transmission increases to a higher or lower degree, depending on the design of the system.
Many proposals are known how an information communication system manually or automatically can be adjusted to variations of the transmission properties of a transmission channel. When the variations are expressed as changes in the signal-noise ratio for the digital signals, the energy of the digital signals in relation to the noise and other interferences could in principle be increased, where possible. This can in principle be brought about, for example, by increasing the effect or durability of the signals. It is, however, not always possible, for technical and/or economic reasons, to increase the signal effect, for example in certain satellite communication systems, and an increase in the durability of the signals, for example according to U.S. Pat. No. 3,700,820, often results in a reduced transmission capacity of the system at poor transmission properties. This can be a disadvantage.
A basically different method of adjusting a communication system to variations in the transmission properties of a transmission channel is to use error correcting coding of the analog information to be transmitted. The error correcting capacity of the coding can manually or automatically be adjusted to the transmission properties, for example according to U.S. Pat. No. 469,125 (Data Transmission System, invented by Alexander H. Frey Jr.). According to these proposals, the error correcting capacity of the system is adjusted automatically to the frequency of digital errors at the transmission by variation of the number of redundant digital signals. Such a variation, however, also implies that the transmission capacity of the system varies as the redundancy of the digital signals varies. Moreover, more and/or more complex coders and decoders are required.
The present invention is based on somewhat different ideas which the inventors Nils Rydbeck and Carl-Erik Sundberg have explicitly described in the following publications by Telecommunication Theory, Lund Institute of Technology, Sweden:
(a) "Techniques for introducing Error Correcting Codes into TDMA Satellite Communication Systems", Technical Report TR-55.
(b) "Pulse Code Modulation with Error Correcting Codes for TDMA Satellite Communication Systems", Technical Report TR-57.
(c) "Further investigations of PCM/TDMA satellite communication systems with error correcting codes", Techincal Report TR-64.
(d) "PCM/TDMA Systems with Error Detecting Codes", Technical Report TR-70.